Wildlife biologists surveying a large proposed concentrating solar power generation facility south of Blythe reported the presence of endangered desert birds on the site in April, but subsequent investigations have turned up no further evidence of the birds. According to documents provided by solar developer BrightSource and made public this week by the California Energy Commission, biologists working on the Rio Mesa Solar Electric Generating Facility site heard elf owls on the site in April, but have since come back empty-handed.

The elf owl, Micrathene whitneyi, has been nearly extirpated from California and has been added to the state's Endangered Species List by the California Department of Fish and Game. It is the world's lightest owl species, with an average body weight of less than one and a half ounces and generally around 5 inches tall.

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The presence of elf owls on the site would be of concern due to the importance of nesting habitat in the California section of their range. In the absence of their preferred breeding habitat, mature saguaro forests, the owls stick to "microphyll woodland" -- the linear forests of mesquite, ironwood, palo verde, and acacia that grow along desert washes -- and destruction of these woodlands by agriculture and tamarisk invasion is thought to account for the species' precarious existence in California.

BrightSource's Rio Mesa Solar project would include two 750-foot power towers surrounded by heliostats, each rated at 250 megawatts' generating capacity, on 4,070 acres of land southeast of Blythe most of which is owned by the Metropolitan Water District. Some conservationists have expressed concern that the heliostats' focusing sunlight on the power towers may pose a threat to birds migrating through the area: a study of the 1980s-era power tower concentrating solar facility at Daggett, California indicated that some birds on the site suffered burn injuries due to the focused sunlight. BrightSource, for its part, downplays those concerns.

The wildlife biologists who surveyed the site for elf owls did so by playing recorded elf owl calls; it's generally accepted by elf owl specialists that any birds in the area will respond within a minute or so of hearing recorded calls. Biologists received responses from elf owls on two separate occasions in the evening of April 15. Subsequent surveys conducted by the contracting biologists produced no further evidence of elf owls on the site, though the biologists did note the presence of brown-crested flycatchers, on the CDFG Watch List; loggerhead shrikes, regarded as a species of concern by both CDFG and the U.S. Fish and Wildlife Service; and a Swainson's hawk, a state Threatened species.

BirghtSource's biologists concluded that the owls detected in April were likely migrating through the site, based on the lack of subsequent detections.

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